Acceleration sensor

ABSTRACT

The acceleration sensor according to the present invention includes a circuit chip having a prescribed circuit built into a front surface thereof; a sensor chip bonded to the front surface of the circuit chip; and a resin package for sealing the circuit chip and the sensor chip, while the sensor chip includes: a membrane arranged to oppose to the front surface of the circuit chip and having a plurality of openings; a piezoresistor formed on a surface of the membrane opposed to the circuit chip; a support section provided on a side opposite to the circuit chip with respect to the membrane and supporting a peripheral edge portion of the membrane; and a weight section provided on the side opposite to the circuit chip with respect to the membrane and integrally held on a central portion of the membrane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an acceleration sensor and asemiconductor device.

2. Description of Related Art

Loading of a sensor (MEMS sensor) employing an MEMS (Micro ElectroMechanical Systems) technique on a portable telephone has recently beenstarted, whereby the MEMS sensor draws increasing attention. Anacceleration sensor for detecting the acceleration of an object is knownas a typical MEMS sensor.

FIG. 10 is a sectional view schematically showing the structure of aconventional acceleration sensor.

The acceleration sensor 101 shown in FIG. 10 includes a circuit chip 104having a circuit for calculating and correcting acceleration, a sensorchip 105 having a piezoresistor (not shown) and a weight 106 of tungstenin a cavity formed of a ceramic package 102 and a shielding plate 103.

The ceramic package 102 has a six-layer structure obtained by laminatingsix ceramic substrates 102A to 102F, for example. The lower threeceramic substrates 102A, 102B and 102C have rectangular shapes of thesame size in plan view. The upper three ceramic substrates 102D, 102Eand 102F have the same outline as the ceramic substrates 102A, 102B and102C in plan view, and are formed with rectangular openings on eachcentral portion. The opening of the ceramic substrate 102D laminated onthe ceramic substrate 102C is smaller than that of the ceramic substrate102E laminated on the ceramic substrate 102D. The opening of the ceramicsubstrate 102E is smaller than that of the ceramic substrate 102Flaminated on the ceramic substrate 102E.

A plurality of pads 107 are arranged on the upper surface of the ceramicsubstrate 102D. The respective pads 107 are electrically connected tothe circuit chip 104 and the sensor chip 105 through bonding wires 108,respectively. Wires 109 extending from the pads 107 are formed on theupper surface of the ceramic substrate 102D. The respective wires 109are connected to an electrode 111 arranged on the lower surface of thelowermost ceramic substrate 102A through via holes 110 verticallypenetrate the lower three ceramic substrates 102A, 102B and 102C.

The shielding plate 103 is bonded to the upper surface of the uppermostceramic substrate 102F to close the opening of the ceramic substrate102F.

The circuit chip 104 is formed of a silicon chip. The circuit chip 105is bonded to the upper surface of the ceramic substrate 102C throughsilver paste, while directing the front surface of a device formingregion thereof upward.

The sensor chip 105 integrally includes a membrane 112, a frame-shapedsupport section 113 connected to a peripheral edge portion of onesurface (lower surface) of the membrane 112 and a weight fixing section114 connected to the central portion of the one surface of the membrane112. The piezoresistor (not shown) is formed on the other surface (uppersurface) of the membrane 112. The support section 113 and the weightfixing section 114 are separated from each other by an annular groove116 having a section in the form of an isosceles trapezoid narrowing asapproaching the membrane 112.

The sensor chip 105 is supported above the circuit chip 104 by chipspacers 115 interposed between the respective corner portions of thesupport section 113 and the front surface of the circuit chip 104 at aprescribed interval with respect to the front surface of the circuitchip 104.

The weight 106 is fixed to the lower surface of the weight fixingsection 114 with an adhesive in a state not in contact with the circuitchip 104, the support section 113 and the chip spacers 115 between thecircuit chip 104 and the sensor chip 105.

When acceleration acts on the acceleration sensor 101 and the weight 106oscillates, the membrane 112 is deformed, and stress acts on thepiezoresistor provided on the membrane 112. The resistivity of thepiezoresistor changes in proportion to the stress acting thereon.Therefore, the acceleration acting on the acceleration sensor 101 can beobtained on the basis of the change in the resistivity of thepiezoresistor.

In the conventional acceleration sensor 101, however, the cost isdisadvantageously increased due to the employment of the ceramic package102. Further, the weight 106 is provided independently of the circuitchip 104 and the sensor chip 105, whereby the acceleration sensor 101 ishard to downsize.

Further, when the weight 106 (membrane 112) oscillates, the membrane 112is distorted, and the distortion may propagate to the peripheral edgeportion of the membrane 112 to crease the peripheral edge portion.

The sensor chip 105 is formed by etching a silicon wafer from the rearsurface side (opposite to the side provided with the membrane 112).Since a thin layer portion on the front surface of the silicon wafermust be left as the membrane 112, the silicon wafer cannot be etchedfrom the front surface side. In a step of manufacturing the conventionalsensor chip 105, therefore, the silicon wafer can be etched only fromthe rear surface side, and a long time is required for the etching.

When a wet etching process is employed for etching the silicon wafer,the silicon wafer is etched in the direction parallel to the rearsurface thereof through the so-called side etching. In order to patternthe silicon wafer into a desired shape, therefore, the shape of a mask(resist pattern) formed on the rear surface of the silicon wafer must bedecided in consideration of both of the etching rate in the directionperpendicular to the rear surface of the silicon wafer and the rate ofthe side etching. Therefore, much labor is required for designing themask.

Further, steps of manufacturing an MEMS sensor include a step ofthinning a silicon wafer. The silicon wafer can be thinned by grindingand/or etching the silicon wafer from the rear surface side (opposite tothe front surface formed with a device). At present, a silicon waferemployed for an MEMS sensor is thinned up to a thickness of about 200 μmto 400 μm in a wafer thinning step.

As shown in FIG. 11, an edge portion of a silicon wafer has a round(convexed) sectional shape. If the silicon wafer is thinned to not lessthan ½ of the original thickness, therefore, the sectional shape of theedge portion is sharpened, and the mechanical strength of the edgeportion is extremely reduced. Consequently, the edge portion of thesilicon wafer may be broken by the so-called edge chipping when thesilicon wafer is handled.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide an accelerationsensor and a semiconductor device reducible in cost and size, and amethod of manufacturing the semiconductor device.

A second object of the present invention is to provide a semiconductordevice capable of preventing the peripheral portion of a membrane fromcreasing.

A third object of the present invention is to provide a method ofmanufacturing a semiconductor device capable of reducing the timerequired for manufacturing the semiconductor device.

A fourth object of the present invention is to provide a method ofmanufacturing a semiconductor device capable of reducing labor requiredfor designing a mask for patterning an etching layer of interest.

A fifth object of the present invention is to provide a method capableof manufacturing a semiconductor device using a thinned silicon waferwithout causing edge chipping of the silicon wafer.

One aspect of the present invention may provide an acceleration sensorincluding a circuit chip having a prescribed circuit built into a frontsurface thereof; a sensor chip bonded to the front surface of thecircuit chip (in a face-to-face state); and a resin package for sealingthe circuit chip and the sensor chip. The sensor chip includes: amembrane arranged to oppose to the front surface of the circuit chip andhaving a plurality of openings; a piezoresistor formed on a surface ofthe membrane opposed to the circuit chip; a support section provided ona side opposite to the circuit chip with respect to the membrane andsupporting a peripheral edge portion of the membrane; and a weightsection provided on the side opposite to the circuit chip with respectto the membrane and integrally held on a central portion of themembrane.

Another aspect of the present invention may provide a semiconductordevice bonded to a front surface of a circuit chip having a prescribedcircuit built thereinto and constituting an acceleration sensor with thecircuit chip. The semiconductor device includes a membrane opposed tothe front surface of the circuit chip and having a plurality ofopenings; a piezoresistor formed on a surface of the membrane opposed tothe circuit chip; a support section provided on a side opposite to thecircuit chip with respect to the membrane and supporting a peripheraledge portion of the membrane; and a weight section provided on the sideopposite to the circuit chip with respect to the membrane and integrallyheld on a central portion of the membrane.

When acceleration acts on the acceleration sensor and the weight sectionoscillates, the membrane is deformed, and stress acts on thepiezoresistor provided on the membrane. When the stress acts on thepiezoresistor, the resistivity of the piezoresistor changes inproportion to the stress, and a signal corresponding to the change ofthe resistivity is input from the sensor chip (semiconductor device) tothe circuit chip. The circuit (prescribed circuit) built into thecircuit chip generates a signal corresponding to the amount of thechange in the resistivity of the piezoresistor, and the signal is outputfrom the circuit chip. Therefore, the acceleration acting on theacceleration sensor can be obtained on the basis of the signal outputfrom the circuit chip.

The membrane and the weight section are formed integrally with eachother, whereby the acceleration sensor can be downsized as compared witha structure including a weight independent of a circuit chip and asensor chip.

The circuit chip and the sensor chip are sealed with the resin package.Therefore, no ceramic package is required that is employed in aconventional acceleration sensor. Consequently, the cost for theacceleration sensor can be reduced.

The acceleration sensor preferably includes a bump interposed betweenthe circuit chip and the sensor chip and connecting the circuit chip andthe sensor chip with each other at a prescribed interval. The circuitchip and the sensor chip can be precisely maintained at the prescribedinterval by interposing the bump between the circuit chip and the sensorchip. Therefore, the range of oscillation of the weight section can beensured in the direction opposed to the circuit chip and the sensorchip. Consequently, the acceleration can be excellently detected.

The bump preferably includes a circuit-chip-side bump projectinglyformed on the front surface of the circuit chip with a Au material; asensor-chip-side pad formed on the surface of the membrane opposed tothe circuit chip with a Au material; and a connecting metal sectionformed of a Sn material for connecting the circuit-chip-side bump andthe sensor-chip-side pad with each other. In other words, the bumpinterposed between the circuit chip and the sensor chip is preferablyformed by connecting the circuit-chip-side bump and the sensor-chip-sidebump of the Au material with each other by the connecting metal sectionof the Sn material. The Sn material has a lower melting point than thatof the Au material, whereby the circuit-chip-side bump and thesensor-chip-side bump can be reliably connected with each other byproviding the Sn material for the connecting metal section to the distalend(s) of the circuit-chip-side bump and/or the sensor-chip-side pad andmelting the Sn material by heating while the circuit-chip-side bump andthe sensor-chip-side pad against are abut each other.

The acceleration sensor preferably includes a lid member provided so asto close a space between the support section and the weight section fromthe side opposite to the membrane. The space between the support sectionand the weight section is closed with the lid member from the sideopposite to the membrane, whereby the material of the resin package canbe prevented from infiltrating into the space between the supportsection and the weight section when the circuit chip and the sensor chipare sealed with the resin package.

The acceleration sensor preferably includes a surface film provided onthe front surface of the circuit chip for regulating the amount ofoscillation of the weight section in the direction opposed to thecircuit chip and the sensor chip. In this case, the amount ofoscillation of the weight section is regulated by the surface film inthe direction opposed to the circuit chip and the sensor chip, wherebythe membrane can be prevented from breakage resulting from excessiveoscillation of the weight section.

The weight section is preferably shaped such that a sectional area takenalong a plane perpendicular to the direction opposed to the circuit chipand the sensor chip increases as separating from the membrane. In thiscase, a surface of the weight section exhibiting the smallest sectionalarea along the plane perpendicular to the direction opposed to thecircuit chip and the sensor chip is connected to the membrane. In theweight section, the sectional area taken along the plane perpendicularto the direction opposed to the circuit chip and the sensor chipincreases as separating from the surface connected to the membrane.Therefore, the weight section reliably oscillates even by smallacceleration in the direction perpendicular to that opposed to thecircuit chip and the sensor chip to deform the membrane. Thus, theacceleration sensor can excellently detect the small acceleration in thedirection perpendicular to that opposed to the circuit chip and thesensor chip.

Still another aspect of the present invention may provide asemiconductor device includes a membrane; and a support section forsupporting a peripheral edge portion of the membrane. The membrane isprovided with a plurality of openings having longitudinal portions.

This semiconductor device may further include a plurality ofpiezoresistors formed on the membrane; and a weight section formed onthe same side as the support section with respect to the membrane andintegrally held on a central portion of the membrane. When accelerationis caused to the weight section, the weight section oscillates tovibrate the membrane, and stress acts on the piezoresistors provided onthe membrane. The resistivity of the piezoresistors changes inproportion to the stress acting thereon. When the change in theresistivity of each piezoresistor is extracted as a signal, thedirection (triaxial direction) and the magnitude of the accelerationacting on the weight section can be obtained on the basis of the signal.In other words, the semiconductor device can be employed for anacceleration sensor, when further including the piezoresistors and theweight section.

Since the plurality of openings having the longitudinal portions areprovided in the membrane, distortion of the membrane caused by vibrationcan be absorbed by deformation of the openings. Therefore, thedistortion of the membrane can be prevented from propagation to theperipheral edge portion, and the peripheral edge portion of the membranecan be prevented from creasing.

The membrane is preferably formed a rectangular opening elongated in onedirection and another rectangular opening elongated in the otherdirection perpendicular to the one direction alternately in the onedirection and the other direction. Thus, distortion of the membrane inany direction can be absorbed by deformation of the rectangular openingelongated in the one direction and/or the rectangular opening elongatedin the other direction. Therefore, the distortion of the membrane can bereliably prevented from propagation to the peripheral edge portion.

A further aspect of the present invention may provide a method ofmanufacturing a semiconductor device, including: a non-etching filmforming step of forming a non-etching film having resistance to anetching solution on a front surface of a silicon wafer; a resistanceelement forming step of forming a piezoresistor on the non-etching film;an opening forming step of forming a plurality of openings for exposingthe silicon wafer in an annular region located between a peripheral edgeportion and a central portion of the unetched film; and an etching stepof removing a portion of the silicon wafer opposed to the annular regionby supplying the etching solution to the silicon wafer through theplurality of openings.

According to the method of manufacturing a semiconductor device, thesemiconductor device can be manufactured. According to the method,further, the sensor chip can be manufactured. Therefore, the method ofmanufacturing a semiconductor device can also be referred to as a methodof manufacturing the sensor chip.

According to the method of manufacturing a semiconductor device, thesilicon wafer can be etched from a rear surface side obviously, and canalso be etched from a front surface side by supplying the etchingsolution through the openings formed in the non-etching film.

When the etching solution is supplied to both of the front and rearsurfaces of the silicon wafer, the etching progresses from both of thefront and rear surfaces of the silicon wafer, whereby the time requiredfor removing the portion of the silicon wafer opposed to the annularregion can be reduced. Consequently, the time required for manufacturingthe sensor chip can be reduced.

The method of manufacturing a semiconductor device preferably furtherincludes a rear surface grinding step of grinding the rear surface ofthe silicon wafer in advance of the etching step. In this case, the rearsurface of the silicon wafer is ground in advance of the etching step,whereby the amount of the silicon wafer etched by the etching solutioncan be reduced. Consequently, the time required for the etching can bereduced.

A further aspect of the present invention may provide a method ofmanufacturing a semiconductor device, including: a mask layer formingstep of forming a mask layer having resistance to an etching solution onan etching layer of interest; a through-hole forming step of forming aplurality of through-holes in the etching layer of interest and the masklayer to penetrate the etching layer of interest and the mask layer inthe laminating direction; and an etching step of patterning the etchinglayer of interest by supplying the etching solution into thethrough-holes from a side of the mask layer and progressing etching in adirection intersecting with the laminating direction from thethrough-holes.

According to this method of manufacturing a semiconductor device, themask layer having resistance to the etching solution is formed on theetching layer of interest. Thereafter, the plurality of through-holesare formed in the etching layer of interest and the mask layer topenetrate the etching layer of interest and the mask layer. Then, theetching solution for etching the etching layer of interest is suppliedinto the plurality of through-holes from the side of the mask layer.Thus, the etching layer of interest is etched through the mask layerhaving the plurality of through-holes. The etching progresses in thedirection intersecting with the laminating direction of the etchinglayer of interest and the mask layer from the side surfaces of therespective through-holes. When the pitch of the through-holes (intervalbetween the through-holes) and the positions thereof are decided inconsideration of only the etching rate in the direction intersectingwith the laminating direction, the etching layer of interest can bepatterned into a desired shape by communicating the plurality ofthrough-holes with one another in the etching layer of interest. Thus,the labor for designing a mask for patterning the etching layer ofinterest can be reduced as compared with a conventional method.

A further aspect of the present invention may provide a method ofmanufacturing a semiconductor device, including: a first wafer thinningstep of thinning a central portion of a silicon wafer by forming arecess on a central portion of a rear surface of the silicon waferopposite to a front surface to be provided with a device; and a secondwafer thinning step of thinning the central portion of the silicon waferand an edge portion around the central portion thereof by etching thefront surface and the rear surface of the silicon wafer after the firstwafer thinning step.

According to the method of manufacturing a semiconductor device, thecentral portion of the silicon wafer is thinned by forming the recess onthe central portion of the rear surface of the silicon wafer (portionincluded in a device forming region on the rear surface of the siliconwafer). Thereafter, the central portion and the edge portion of thesilicon wafer are thinned by etching the front surface and the rearsurface of the silicon wafer. The etching progresses at the same rate onthe front surface and the central and edge portions of the rear surfaceof the silicon wafer. When the central portion of the silicon wafer isthinned to a desired thickness, the edge portion of the silicon waferhas a thickness greater than that of the central portion by the depth ofthe recess. Thus, mechanical strength can be ensured on the edge portionof the silicon wafer, whereby the silicon wafer can be prevented fromedge chipping. Therefore, a semiconductor device employing the thinnedsilicon wafer can be manufactured without causing edge chipping of thesilicon wafer.

The above and other objects, features and effects of the presentinvention will become more apparent from the following detaileddescription of the present invention with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically showing the structure of anacceleration sensor according to a first embodiment of the presentinvention;

FIG. 2 illustrates a part of a membrane of a sensor chip according tothe first embodiment;

FIG. 3A is a schematic sectional view for illustrating a step ofmanufacturing the sensor chip according to the first embodiment;

FIG. 3B is a schematic sectional view showing a step next to that shownin FIG. 3A;

FIG. 3C is a schematic sectional view showing a step next to that shownin FIG. 3B;

FIG. 3D is a schematic sectional view showing a step next to that shownin FIG. 3C;

FIG. 3E is a schematic sectional view showing a step next to that shownin FIG. 3D;

FIG. 3F is a schematic sectional view showing a step next to that shownin FIG. 3E;

FIG. 3G is a schematic sectional view showing a step next to that shownin FIG. 3F;

FIG. 4 is a sectional view schematically showing the structure of asemiconductor device according to a second embodiment of the presentinvention;

FIG. 5 is a plan view showing a part of a membrane of the semiconductordevice according to the second embodiment in an enlarged manner;

FIG. 6 is a plan view showing another shape of openings formed in themembrane;

FIG. 7 is a sectional view schematically showing the structure of asemiconductor device according to a third embodiment of the presentinvention;

FIG. 8A is a schematic sectional view for illustrating a step ofmanufacturing the semiconductor device according to the secondembodiment;

FIG. 8B is a schematic sectional view showing a step next to that shownin FIG. 8A;

FIG. 8C is a schematic sectional view showing a step next to that shownin FIG. 8B;

FIG. 9A is a schematic sectional view for illustrating a method ofmanufacturing a semiconductor device according to a fourth embodiment ofthe present invention;

FIG. 9B illustrates a schematic sectional view and a schematicperspective view showing a step next to that shown in FIG. 9A;

FIG. 9C is a schematic sectional view showing a step next to that shownin FIG. 9B;

FIG. 10 is a sectional view schematically showing the structure of aconventional acceleration sensor; and

FIG. 11 is a schematic side elevational view showing the shape of anedge portion of a conventional thinned silicon wafer.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention are now described in detail withreference to the accompanying drawings.

First Embodiment

FIG. 1 is a sectional view schematically showing the structure of anacceleration sensor according to a first embodiment of the presentinvention.

The acceleration sensor 1 is a piezoresistance type acceleration sensor.The acceleration sensor 1 has a chip-on-chip structure formed bysuperposing and bonding a circuit chip 2 and a sensor chip 3 serving asa semiconductor device with and to each other.

The circuit chip 2 has a generally rectangular shape in plan view. Thecircuit chip 2 is die-bonded to an island section 6 of a lead frame 5 ina face-up state directing the front surface 4 thereof upward.

A circuit (not shown) generating a signal corresponding to a change inthe resistivity of piezoresistors 16 described later and the like arebuilt into a surface layer portion of the circuit chip 2 including thefront surface 4.

A chip bonding region is set at the central portion of the front surface4 of the circuit chip 2, so that the sensor chip 3 is bonded thereto. Aplurality of circuit-chip-side bumps 7 are arranged on the chip bondingregion along the peripheral edge of the chip bonding region at intervalsfrom one another. The respective circuit-chip-side bumps 7 areprojectingly formed on the front surface 4 using a Au material. Asurface film 8 of polyimide is formed on the front surface 4 of thecircuit chip 2. The chip bonding region is covered with the surface film8.

On the front surface 4 of the circuit chip 2, a plurality of externalconnection pads 9 are provided on the peripheral edge portionsurrounding the chip bonding region. The external connection pads 9 areelectrically connected (wire-bonded) to lead portions 11 of the leadframe 5 through bonding wires 10.

The sensor chip 3 has a generally rectangular outline smaller than thecircuit chip 2 in plan view. This sensor chip 3 is bonded to the chipbonding region of the front surface 4 of the circuit chip 2 in aface-down state directing the front surface 12 thereof downward.

The sensor chip 3 integrally includes a membrane 13 opposed to the frontsurface 4 of the circuit chip 2, a support section 14 provided on theside opposite to the circuit chip 2 with respect to the membrane 13 forsupporting the peripheral edge portion of the membrane 13, and a weightsection 15 provided on the side opposite to the circuit chip 2 withrespect to the membrane 13 and held at the central portion of themembrane 13.

The surface of the membrane 13 opposed to the circuit chip 2 forms thefront surface 12 of the sensor chip 3. The membrane 13 is made of SiO₂(silicon oxide), for example, and formed to have a thickness of 1 to 10μm. On the surface of the membrane 13 opposed to the circuit chip 2, aplurality of (e.g. 16) piezoresistors 16 are formed on an annular regionlocated between the peripheral edge portion and the central portionthereof, as shown in FIG. 2. Further, a large number of rectangularopenings 17 are formed in the annular region. Thus, the annular regionlocated between the peripheral edge portion and the central portion ofthe membrane 13 is in the form of a mesh.

Wires 18 connected to the respective piezoresistors 16 are also formedon the surface of the membrane 13 opposed to the circuit chip 2. Au isemployed as the material for the wires 18. Each wire 18 is covered witha wire protective film 19 of SiN (silicon nitride). Each wire 18 extendstoward the peripheral edge portion of the membrane 13, and has a pad 21(see FIG. 3G) exposed from a pad opening 20 (see FIG. 3G) formed in thewire protective film 19 in the peripheral edge portion. Each pad 21 isarranged on a position opposed to each circuit-chip-side bump 7 of thecircuit chip 2.

When the circuit chip 2 and the sensor chip 3 are bonded to each otheras shown in FIG. 1, each circuit-chip-side bump 7 of the circuit chip 2and each pad 21 of the sensor chip 3 are opposed to each other whileabutting the top faces thereof against each other, and connected to eachother through a connecting metal section 22 of a Sn material.

Before the circuit chip 2 and the sensor chip 3 are bonded to eachother, the Sn material for forming the connecting metal section 22 isapplied to the top face of each circuit-chip-side bump 7 of the circuitchip 2. When the circuit chip 2 and the sensor chip 3 are bonded to eachother and heat-treated while abutting each circuit-chip-side bump 7 ofthe circuit chip 2 and each pad 21 of the sensor chip 3 against eachother, the Sn material applied to the top face of each circuit-chip-sidebump 7 is melted. Thus, the connecting metal section 22 is formedbetween each circuit-chip-side bump 7 and each pad 21. Eachcircuit-chip-side bump 7 and each pad 21 can be reliably connected toeach other by the connecting metal section 22.

When each circuit-chip-side bump 7 and each pad 21 are connected to eachother through the connecting metal section 22, the circuit chip 2 andthe sensor chip 3 are electrically connected with each other througheach bump formed by connecting each circuit-chip-side bump 7 and eachpad 21 to each other through the connecting metal section 22, andmechanically connected with each other while keeping a prescribedinterval therebetween.

The support section 14 and the weight section 15 are made of Si, andseparated from each other by a rectangular annular groove section 23having a section in the form of an isosceles trapezoid narrowing upward.

The support section 14 is in the form of an annulus, whose sectionalarea taken along a plane parallel to the surface connected to themembrane 13 monotonically increases as separating from the membrane 13.The outer side surface of the support section 14 is perpendicular to thesurface connected to the membrane 13. The inner side surface of thesupport section 14 is inclined at an angle of 54.7° with respect to thesurface connected to the membrane 13.

The weight section 15 is in the form of an isosceles trapezoid insection, whose sectional area taken along a plane parallel to thesurface connected to the membrane 13 monotonically increases asseparating from the membrane 13. The side surfaces of this weightsection 15 are also inclined at the angle of 54.7° with respect to thesurface connected to the membrane 13.

A lid member 24 is provided on the rear surface of the sensor chip 3opposite to the front surface 4 to close the groove section 23 locatedbetween the support section 14 and the weight section 15. The circuitchip 2 and the sensor chip 3 are sealed with a resin package 25 alongwith the lead frame 5 and the bonding wires 10. The lid member 24 is soprovided that the material of the resin package 25 can be prevented frominfiltrating into the groove section 23 when the chips 2 and 3 aresealed with the resin package 25. Parts of the lead portions 11 of thelead frame 5 are exposed from the resin package 25 to function as outerconnecting portions (outer lead portions) for a printed wiring board orthe like.

When acceleration acts on the acceleration sensor 1 and the weightsection 15 oscillates, the membrane 13 is deformed, and stress acts onthe piezoresistors 16 provided on the membrane 13. When the stress actson the piezoresistors 16, the resistivity thereof changes in proportionto the stress, and a signal corresponding to the change of theresistivity is input from the sensor chip 3 to the circuit chip 2. Thecircuit built into the circuit chip 2 generates a signal correspondingto the amount of the change in the resistivity of the piezoresistors 16,and the signal is output to the lead portions 11 that functions as theexternal connecting portions through the external connection pads 9 andthe bonding wires 10. Therefore, the direction (triaxial direction) andthe magnitude of the acceleration acting on the acceleration sensor 1can be obtained on the basis of the signal output from the lead portions11.

FIGS. 3A to 3G are schematic sectional views for illustrating steps ofmanufacturing the sensor chip 3.

For manufacture the sensor chip 3, a silicon wafer 31 having a thicknessof 625 μm, for example, is employed. First, a silicon oxide film 32serving as a non-etching film is formed on the front surface of thesilicon wafer 31 by plasma treatment, as shown in FIG. 3A (non-etchingfilm forming step).

Then, the plurality of piezoresistors 16 are formed on the silicon oxidefilm 32 using TiN (titanium nitride), as shown in FIG. 3B (resistanceelement forming step).

Then, a resistance protective film 33 of SiO₂ is formed to cover eachpiezoresistor 16, as shown in FIG. 3C. Each resistance protective film33 is partially removed, so that a connection opening 34 for partiallyexposing the piezoresistor 16 is formed in each resistance protectivefilm 33.

Thereafter, the wire 18 is formed over each resistance protective film33 and the silicon oxide film 32, as shown in FIG. 3D. Each wire 18 isconnected to the corresponding piezoresistor 16 through the connectionopening 34 formed in the resistance protective film 33.

Then, the wire protective film 19 of SiN is formed to cover each wire18, as shown in FIG. 3E. Then, the pad opening 20 for partially exposingthe wire 18 is formed in each wire protective film 19.

Then, the large number of rectangular openings 17 are formed in theannular region located between the peripheral edge portion and thecentral portion of the silicon oxide film 32 (region opposed to thegroove section 23 formed later), as shown in FIG. 3F (opening formingstep). Consequently, the silicon wafer 31 is exposed through the largenumber of openings 17.

Then, the rear surface of the silicon wafer 31 is ground by a thicknessof 50 μm, for example (rear surface grinding step).

Then, an etching solution having ability to etch the silicon wafer 31 issupplied to both of the front and rear surfaces of the silicon wafer 31.A solution prepared by heating an aqueous solution of TMAH (tetramethylammonium hydroxide) having a concentration of 50% to 80° C., forexample, can be employed as such an etching solution.

The etching solution supplied to the front surface of the silicon wafer31 is supplied to the silicon wafer 31 through each opening 17. Thus,the silicon wafer 31 is pyramidally etched from the portion facing eachopening 17, as shown in FIG. 3G (etching step). The etching progressesin the thickness direction of the silicon wafer 31 (vertical direction)and the direction parallel to the front surface thereof (transversedirection) while each side surface of each pyramidal recess 35 formed bythe etching maintains the inclination of 54.7° with respect to the frontsurface of the silicon wafer 31. When the recesses 35 formed under theadjacent openings 17 are linked with one another, the transverse etchingrapidly progresses, and the portions located under the piezoresistors 16are also etched. On the other hand, the etching solution is alsosupplied to the rear surface of the silicon wafer 31, whereby thesilicon wafer 31 is vertically etched at a generally uniform in-planespeed also from the rear surface side (rear surface etching step).Therefore, a portion of the silicon wafer 31 opposed to an annularregion located between the peripheral edge portion and the centralportion of the silicon oxide film 32 is completely removed over thethickness direction, and the annular groove section 23 shown in FIG. 1is formed in the silicon wafer 31. Consequently, the silicon oxide film32 defines the membrane 13, and the silicon wafer 31 is divided into thesupport section 14 supporting the peripheral edge portion of themembrane 13 and the weight section 15 held on the central portion of themembrane 13 by the groove section 23. Thus, the sensor chip 3 shown inFIG. 1 is obtained.

The wires 18 are made of Au, so that the wires 18 are not corroded bythe etching solution despite the exposure through the pad openings 20.Therefore, when the silicon wafer 31 is etched by the etching solution,there is no need to form protective films for protecting the wires 18exposed through the pad openings 20 against the etching solution.

As hereinabove described, the sensor chip 3 face-to-facedly bonded tothe front surface 4 of the circuit chip 2 includes the membrane 13, thesupport section 14 supporting the peripheral edge portion of themembrane 13 and the weight section 15 held on the central portion of themembrane 13. The membrane 13 and the weight section 15 are formedintegrally with each other. Thus, the acceleration sensor 1 can bedownsized as compared with a structure including a weight sectionindependent of a circuit chip and a sensor chip.

Further, the circuit chip 2 and the sensor chip 3 are sealed with theresin package 25. Therefore, the acceleration sensor 1 requires noceramic package that is used for a conventional acceleration sensor.Consequently, the cost for the acceleration sensor 1 can be reduced.

The groove section 23 located between the support section 14 and theweight section 15 is closed by the lid member 24 from the side oppositeto the membrane 13. Therefore, the material of the resin package 25 canbe prevented from infiltrating into the space between the supportsection 14 and the weight section 15 when the circuit chip 2 and thesensor chip 3 are sealed with the resin package 25.

Further, the bumps formed by connecting the circuit-chip-side bumps 7and the pads 21 with one another by the connecting metal sections 22 areinterposed between the circuit chip 2 and the sensor chip 3. Thus, thecircuit chip 2 and the sensor chip 3 can be precisely maintained at aprescribed interval. Therefore, the range of oscillation of the weightsection 15 can be ensured in the direction opposed to the circuit chip 2and the sensor chip 3. Consequently, the acceleration can be excellentlydetected.

In addition, the surface film 8 of polyimide is formed on the frontsurface 4 of the circuit chip 2. The surface film 8 can prevent theweight section 15 from excessively oscillating in the direction opposedto the circuit chip 2 and the sensor chip 3. Therefore, the membrane 13can be prevented from breakage or the like resulting from excessiveoscillation of the weight section 15.

In the weight section 15, the surface having the smallest sectional areataken along the plane perpendicular to the direction opposed to thecircuit chip 2 and the sensor chip 3 is connected to the membrane 13.The sectional area of the weight section 15 taken along the planeperpendicular to the direction opposed to the circuit chip 2 and thesensor chip 3 increases as separating from the surface connected to themembrane 13. Therefore, the weight section 15 reliably oscillates evenby small acceleration in the direction perpendicular to that opposed tothe circuit chip 2 and the sensor chip 3 to deform the membrane 13.Thus, the acceleration sensor 1 can excellently detect the smallacceleration in the direction perpendicular to that opposed to thecircuit chip 2 and the sensor chip 3.

In the steps of manufacturing the sensor chip 3, the silicon oxide film32 serving as a non-etching film having resistance to the etchingsolution is formed on the front surface of the silicon wafer 31, and thelarge number of openings 17 exposing the silicon wafer 31 are formed inthe annular region located between the peripheral edge portion and thecentral portion of the silicon oxide film 32. Therefore, the siliconwafer 31 can be etched from the front surface side by supplying theetching solution to the front surface of the silicon wafer 31 throughthe large number of openings 17.

According to the first embodiment, the etching solution is supplied toboth of the front and rear surfaces of the silicon wafer 31. Therefore,the etching progresses from both of the front and rear surfaces of thesilicon wafer 31, whereby the time necessary for removing the portion ofthe silicon wafer 31 opposed to the annular region can be reduced.

Further, the rear surface of the silicon wafer 31 is ground in advanceof the etching step for the silicon wafer 31. Therefore, the amount ofthe silicon wafer 31 etched by the etching solution can be reduced, andthe time required for this etching can be further reduced.

While the rectangular openings 17 are formed in the silicon oxide film32 (membrane 13) according to the first embodiment, the openings 17 arenot limited to the rectangular shape, but may alternatively have acircular shape. In this case, the groove section 23 has a section in theform of an isosceles trapezoid narrowing upward, and the weight section15 is in the form of a truncated cone having a monotonically increasingsectional area taken along a plane parallel to the surface connected tothe membrane 31.

The wires 18 may be omitted. In this case, the parts of thepiezoresistors 16 exposed from the connecting openings 34 formed in theresistance protective film 33 may be used as pads connected to thecircuit-chip-side bumps 7.

While the silicon oxide film 32 is formed on the front surface of thesilicon wafer 31, a silicon nitride film may alternatively be formed byplasma treatment in place of the silicon oxide film 32. In this case,SiN is employed as the material for the membrane 13.

The aforementioned method of manufacturing the sensor chip 3 is notlimited to the sensor chip 3 employed for the acceleration sensor 1, butis also applicable to a method of manufacturing a sensor chip(semiconductor device) employed for a piezoresistance type semiconductorpressure sensor for detecting the pressure of a gas or the like.

Second Embodiment

FIG. 4 is a sectional view schematically showing the structure of asemiconductor device according to a second embodiment of the presentinvention.

This semiconductor device 40 is employed for an acceleration sensor fordetecting the acceleration of an object, for example. The semiconductordevice 40 includes a membrane 41, a support section 42 provided on thelower surface of the membrane 41 for supporting the peripheral edgeportion of the membrane 41, and a weight section 43 provided on thelower surface of the membrane 41 and held on the central portion of themembrane 41.

The membrane 41 is made of SiO₂ (silicon oxide), for example, andgenerally in the form of a rectangle having a thickness of 1 to 10 μm inplan view. A plurality of (e.g. 16) piezoresistors 44 are formed on themembrane 41. A large number of openings 45 are formed in a quadrangularannular region located between the peripheral edge portion and thecentral portion of the membrane 41.

The support section 42 and the weight section 43 are made of Si, andseparated from each other by a groove section 46 having a quadrangularannular shape in plan view. This groove section 46 has a section in theform of an isosceles trapezoid narrowing upward as separating from themembrane 41.

The support section 42 is in the form of a quadrangular annulus in planview, whose sectional area taken along a plane parallel to the surfaceconnected to the membrane 41 monotonically increases as separating fromthe membrane 41. The outer side surfaces of the support sections 42 areperpendicular to the surface connected to the membrane 41. On the otherhand, the inner side surfaces of the support section 42 are inclined atan angle of 54.7° with respect to the surface connected to the membrane41.

The weight section 43 is in the form of an isosceles trapezoid insection widening as separating from the membrane 41. Each side surfaceof the weight section 43 is inclined at the angle of 54.7° with respectto the surface connected to the membrane 41.

When acceleration acts on this semiconductor device 40 and the weightsection 43 oscillates, the membrane 41 is deformed, and stress acts oneach piezoresistor 44 provided on the membrane 41. The resistivity ofthe piezoresistor 44 changes in proportion to the stress acting thereon.When the amount of the change in the resistivity of each piezoresistor44 is extracted as a signal, the direction (triaxial direction) and themagnitude of the acceleration acting on the weight section 43(semiconductor device 40) can be obtained on the basis of this signal.

FIG. 5 is a plan view showing a part of the membrane 41 in an enlargedmanner.

As shown in FIG. 5, the large number of openings 45 formed in themembrane 41 include rectangular openings 45A elongated in one directionand rectangular openings 45B elongated in the other directionperpendicular to the one direction. The openings 45A and 45B arealternately formed in the one and the other directions. In other words,the rectangular openings 45A elongated in the one direction and therectangular openings 45B elongated in the other direction perpendicularto the one direction are alternately formed in the membrane 41 in theone and the other directions.

According to this structure, distortion of the membrane 41 caused byvibration can be absorbed by the rectangular openings 45A elongated inthe one direction and/or the rectangular openings 45B elongated in theother direction regardless of the direction of the distortion.Therefore, the distortion can be reliably prevented from propagation tothe peripheral edge portion of the membrane 41. Consequently, theperipheral edge portion of the membrane 41 can be prevented fromcreasing.

The semiconductor device 40 having the aforementioned structure ismanufactured in the following manner:

A silicon wafer is employed for manufacturing the semiconductor device40. First, a silicon oxide film having resistance to an etching solutioncapable of etching the silicon wafer is formed on the front surface ofthe silicon wafer by plasma treatment. Then, the plurality ofpiezoresistors 44 are formed on the silicon oxide film by TiN (titaniumnitride). Further, wires connected to the piezoresistors 44 and the likeare formed. Thereafter the large number of openings 45 are formed in thesilicon oxide film. Thus, the silicon wafer is exposed through the largenumber of openings 45.

Then, the rear surface of the silicon wafer is ground if necessary, andan etching solution having ability to etch the silicon wafer is suppliedto both of the front and rear surfaces of the silicon wafer. A solutionprepared by heating an aqueous solution of TMAH (tetramethyl ammoniumhydroxide) having a concentration of 50% to 80° C., for example, can beemployed as such an etching solution. The etching solution supplied tothe front surface of the silicon wafer is supplied to the silicon waferthrough each opening 45. Thus, the silicon wafer is etched in the formof a triangle in section from the portion facing each opening 45. Theetching progresses in the thickness direction of the silicon wafer(vertical direction) and the direction parallel to the front surfacethereof (transverse direction) while each side surface of each recessformed by the etching maintains the inclination of 54.7° with respect tothe front surface of the silicon wafer. When the recesses located underthe adjacent openings 45 are linked with one another, the transverseetching rapidly progresses. On the other hand, the etching solution isalso supplied to the rear surface of the silicon wafer, whereby thesilicon wafer is vertically etched at a generally uniform in-plane speedalso from the side of the rear surface. Therefore, a portion of thesilicon wafer opposed to an annular region located between theperipheral edge portion and the central portion of the silicon oxidefilm (membrane 41) is completely removed over the thickness direction,and the annular groove section 46 is formed in the silicon wafer.Consequently, the silicon oxide film defines the membrane 41, and thesilicon wafer is divided into the support section 42 supporting theperipheral edge portion of the membrane 41 and the weight section 43held on the central portion of the membrane 41 by the groove section 46.Thus, the semiconductor device 40 shown in FIG. 4 is obtained.

While the structure that the rectangular openings 45 are formed in themembrane 41 is described in the second embodiment, a large number ofL-shaped (hooked) openings 45 may alternatively be formed in themembrane 41, as shown in FIG. 6. In other words, the openings 45 formedin the membrane 41 may have at least partially longitudinal portions.When the large number of openings 45 having longitudinal portions areformed in the membrane 41, distortion of the membrane 41 caused byvibration can be absorbed by deformation of the openings 45. Therefore,the distortion can be prevented from propagation to the peripheral edgeportion of the membrane 41, and the peripheral edge portion of themembrane 41 can be prevented from creasing.

The structure that the openings 45 having longitudinal portions areformed in the membrane 41 is not limited to the acceleration sensor, butis also applicable to a semiconductor device employed for apiezoresistance type pressure sensor including a piezoresistor whoseresistivity is changed by deformation of a membrane resulting from thepressure of a gas or the like, for example. The structure is furtherapplicable to a semiconductor device employed for a capacitance typepressure sensor including a capacitor whose capacitance is changed bydeformation of a membrane resulting from the pressure of a gas or thelike or for a microphone. In other words, the structure is widelyapplicable to a semiconductor device including a membrane.

Third Embodiment

FIG. 7 is a sectional view schematically showing the structure of asemiconductor device according to a third embodiment of the presentinvention.

This semiconductor device 70 is employed for an acceleration sensor fordetecting the acceleration of an object. The semiconductor device 70includes a membrane 71, a support section 72 provided on the lowersurface of the membrane 71 for supporting the peripheral edge portion ofthe membrane 71, and a weight section 73 provided on the lower surfaceof the membrane 71 and held on the central portion of the membrane 71.

The membrane 71 is made of SiO₂ (silicon oxide), for example, andgenerally in the form of a rectangle having a thickness of 1 to 10 μm inplan view. A plurality of (e.g. 16) piezoresistors (not shown) areformed on this membrane 71. A large number of openings 74 are formed ina quadrangular annular region located between the peripheral edgeportion and the central portion of the membrane 71.

The support section 72 and the weight section 73 are made of Si, andseparated from each other by a groove section 75 having a quadrangularannular shape in plan view of a rectangular section.

The support section 72 is formed in a quadrangular annular shape in planview, having the outer and inner side surfaces perpendicular to themembrane 71.

The weight section 73 is in the form of a prism having the same heightto that of the support section 72.

When acceleration acts on the semiconductor device 70 and the weightsection 73 oscillates, the membrane 71 is deformed, and stress acts oneach piezoresistor provided on the membrane 71. The resistivity of thepiezoresistor changes in proportion to the stress acting thereon. Whenthe amount of the change in the resistivity of each piezoresistor isextracted as a signal, the direction (triaxial direction) and themagnitude of the acceleration acting on the weight section 73(semiconductor device 70) can be obtained on the basis of this signal.

FIGS. 8A to 8C are schematic sectional views for illustrating steps ofmanufacturing the semiconductor device 70.

A silicon wafer 81 is employed for manufacturing the semiconductordevice 70. First, a silicon oxide film 82 serving as a mask layer havingresistance to an etching solution capable of etching the silicon wafer81 is formed on the front surface of the silicon wafer 81 by plasmatreatment, as shown in FIG. 8A (mask layer forming step).

Then, a large number of through-holes 83 are formed in regions(corresponding to the groove section 75 shown in FIG. 7) havingquadrangular annular shapes in plan view, located between the peripheraledge portions and the central portions of the silicon wafer 81 and thesilicon oxide film 82 by dry etching to penetrate the silicon wafer 81and the silicon oxide film 82 in the stacking direction, as shown inFIG. 8B.

Then, the etching solution capable of etching the silicon wafer 81 issupplied into each through-hole 83 from the side of the silicon oxidefilm 82 (etching step). A solution prepared by heating an aqueoussolution of TMAH (tetramethyl ammonium hydroxide) having a concentrationof 50% to 80° C., for example, can be employed as such an etchingsolution. Each through-hole 83 penetrate the silicon wafer 81, wherebyetching of the silicon wafer 81 progresses in the direction parallel tothe silicon oxide film 82 from the side surface of each through-hole 83when the etching solution is supplied to each through-hole 83, as shownin FIG. 8C. When this etching progresses, the quadrangular annularregion located between the peripheral edge portion and the centralportion of the silicon wafer 81 is completely removed over the thicknessdirection, and the groove section 75 having the quadrangular annularshape in plan view is formed in the silicon wafer 81 as shown in FIG. 7.Consequently, the silicon oxide film 82 defines the membrane 71, and thesilicon wafer 81 is divided into the support section 72 supporting theperipheral edge portion of the membrane 71 and the weight section 73held on the central portion of the membrane 71 by the groove portion 75.Thus, the semiconductor device 70 shown in FIG. 7 is obtained.

According to this method of manufacturing a semiconductor device, asdescribed hereinabove, the silicon oxide film 82 having resistance tothe etching solution is formed on the silicon wafer 81 serving as anetching layer of interest. Thereafter, the plurality of through-holes 83are formed in the silicon wafer 81 and the silicon oxide film 82 topenetrate the silicon wafer 81 and the silicon oxide film 82. Then, theetching solution for etching the silicon wafer 81 is supplied into thethrough-holes 83 from the side of the silicon oxide film 82. Thus, thesilicon wafer 81 is etched through the silicon oxide film 82 serving asa mask, having the plurality of through-holes 83. This etchingprogresses in the direction parallel to the silicon oxide film 82(direction intersecting with the laminating direction of the siliconwafer 81 and the silicon oxide film 82) from the side surfaces of eachthrough-hole 83. When the pitch of the through-holes 83 (intervalbetween the through-holes 83) and the positions thereof are decided inconsideration of only the etching rate in the direction parallel to thesilicon oxide film 82, the silicon wafer 81 can be patterned into adesired shape (shape of the groove section 75: quadrangular annularshape in plan view according to this embodiment) by communicating theplurality of through-holes 83 with one another in the silicon wafer 81.Thus, the labor for designing a mask for patterning the silicon wafer 81can be reduced as compared with a conventional method.

While the silicon oxide film 82 is formed on the front surface of thesilicon wafer 81 according to the third embodiment, a silicon nitridefilm may alternatively be formed by plasma treatment in place of thesilicon oxide film 82. In this case, SiN (silicon nitride) is employedas the material for the membrane 71.

The groove section 75 is not limited to the quadrangular annular shapein plan view, but may alternatively have an annular shape in plan view,for example.

The aforementioned method of manufacturing a semiconductor device is notlimited to the acceleration sensor, but is also applicable to varioustypes of semiconductor devices such as a piezoresistance typesemiconductor pressure sensor for detecting the pressure of a gas or thelike, for example.

Fourth Embodiment

FIGS. 9A to 9C are schematic diagrams for illustrating a method ofmanufacturing a semiconductor device according to a fourth embodiment ofthe present invention.

According to the method of manufacturing a semiconductor device, asilicon wafer W is first prepared, as shown in FIG. 9A. In this siliconwafer W, a central section 91 is a region employed for manufacturing asemiconductor device, and an edge section 92 around the central section91 is a region not employed for manufacturing the semiconductor device.In the central section 91 of the silicon wafer W, a device such as atransistor is formed on a surface layer portion closer to the frontsurface 93.

Then, a recess 95 shaped by entirely recessing a portion included in thecentral section 91 of the rear surface 94 opposite to the front surface93 is formed on the rear surface 94 of the silicon wafer W, as shown inFIG. 9B. Thus, the central section 91 of the silicon wafer W is thinnedto a thickness (400 μm) of about ⅔ of the original thickness (625 μm),for example (first wafer thinning step). The recess 95 may be formedeither by grinding or by etching.

Thereafter an etching solution is supplied to the front surface 93 andthe rear surface 94 of the silicon wafer W for etching the front surface93 and the rear surface 94 of the silicon wafer W (second wafer thinningstep). The etching progresses at the same rate on the front surface 93and the rear surface 94 (including the recess 95) of the silicon waferW. When the central section 91 of the silicon wafer W is thinned to adesired thickness (300 μm, for example), therefore, the edge section 92of the silicon wafer W has a thickness larger than that of the centralsection 91 by the depth of the recess 95, as shown in FIG. 9C.

Thus, mechanical strength can be ensured on the edge section 92 of thesilicon wafer W by leaving a relatively large thickness in the edgesection 92 of the silicon wafer W, whereby the silicon wafer W can beprevented from edge chipping. Therefore, a semiconductor device can bemanufactured using the thinned silicon wafer W without causing edgechipping when the silicon wafer W is handled.

While the illustrative embodiments of the present invention aredescribed in detail, these are mere specific examples for clarifying thetechnical contents of the present invention and it is not to beconstrued limitative thereto. The spirit and the scope of the inventionare only limited by the claims appended hereto.

This application claims priority of Japanese Patent Applications Nos.2006-219610 and 2006-219611 filed on Aug. 11, 2006, Japanese PatentApplications Nos. 2006-333281 and 2006-333282 filed on Dec. 11, 2006 andJapanese Patent Application No. 2006-355713 filed on Dec. 28, 2006, thedisclosures of which are herein incorporated by reference in theirentirely.

1. An acceleration sensor comprising: a circuit chip having a prescribedcircuit built into a front surface thereof; a sensor chip bonded to thefront surface of the circuit chip; and a resin package for sealing thecircuit chip and the sensor chip, the sensor chip comprising: a membranearranged to oppose to the front surface of the circuit chip and having aplurality of openings; a piezoresistor formed on a surface of themembrane opposed to the circuit chip; a support section provided on aside opposite to the circuit chip with respect to the membrane andsupporting a peripheral edge portion of the membrane; a weight sectionprovided on the side opposite to the circuit chip with respect to themembrane and integrally held on a central portion of the membrane; and alid member provided so as to close a space between the support sectionand the weight section from a side opposite to the membrane, wherein asurface of the lid member facing away from the circuit chip is coveredwith the resin package.
 2. The acceleration sensor according to claim 1,further comprising a bump interposed between the circuit chip and thesensor chip and connecting the circuit chip and the sensor chip witheach other at a prescribed interval.
 3. The acceleration sensoraccording to claim 2, wherein the bump comprises: a circuit-chip-sidebump projectingly formed on the front surface of the circuit chip withan Au material; a sensor-chip-side pad formed on the surface of themembrane opposed to the circuit chip with a Au material; and aconnecting metal section formed of a Sn material for connecting thecircuit-chip-side bump and the sensor-chip-side pad with each other. 4.The acceleration sensor according to claim 3, wherein a wire connectedto the piezoresistor is formed on the surface of the membrane opposed tothe circuit chip, and a part of the wire defines the sensor-chip-sidepad connected to the circuit-chip-side bump.
 5. The acceleration sensoraccording to claim 3, wherein the membrane is in a form of a mesh havinga plurality of openings.
 6. The acceleration sensor according to claim1, further comprising a surface film provided between the front surfaceof the circuit chip and a surface of the membrane opposed to the circuitchip for regulating an amount of oscillation of the weight section in adirection in which the circuit chip and the sensor chip are opposed toeach other.
 7. The acceleration sensor according to claim 1, wherein theweight section is shaped such that a sectional area taken along a planeperpendicular to the direction opposed to the circuit chip and thesensor chip increases as separating from the membrane.
 8. Theacceleration sensor according to claim 1, wherein a surface of theweight section facing away from the circuit chip is in contact with thelid member.
 9. The acceleration sensor according to claim 1, wherein awire connected to the piezoresistor is formed on the surface of themembrane opposed to the circuit chip.
 10. The acceleration sensoraccording to claim 9, wherein the wire is covered with a wire protectivefilm.
 11. A semiconductor device bonded to a front surface of a circuitchip having a prescribed circuit built thereinto and constituting anacceleration sensor with the circuit chip, the semiconductor devicecomprising: a membrane arranged to oppose to a front surface of thecircuit chip and being in a form of a mesh having a plurality ofopenings; a piezoresistor formed on a surface of the membrane opposed tothe circuit chip; a support section provided on a side opposite to thecircuit chip with respect to the membrane and supporting a peripheraledge portion of the membrane; and a weight section provided on the sideopposite to the circuit chip with respect to the membrane and integrallyheld on a central portion of the membrane; wherein the weight section isshaped such that a sectional area taken along a plane perpendicular to adirection in which the weight section is opposed to the circuit chipincreases as separating from the membrane.
 12. The semiconductor deviceaccording to claim 11, further comprising a surface film providedbetween the front surface of the circuit chip and a surface of themembrane opposed to the circuit chip for regulating an amount ofoscillation of the weight section in the direction in which the weightsection is opposed to the circuit chip.
 13. The semiconductor deviceaccording to claim 11, wherein a wire connected to the piezoresistor isformed on the surface of the membrane opposed to the circuit chip. 14.The semiconductor device according to claim 13, wherein the wire iscovered with a wire protective film.
 15. The semiconductor deviceaccording to claim 13, further comprising: a circuit-chip-side bumpprojectingly formed on the front surface of the circuit chip with an Aumaterial; a sensor-chip-side pad connected to the circuit-chip-side bumpand formed with a part of the wire; and a connecting metal sectionformed of an Sn material for connecting the circuit-chip-side bump andthe sensor-chip-side pad with each other.